Last night my Nexus 7 shut down automatically due to a battery draining app. This morning I found the device with a black screen and switched off. While connecting it to a charger I tried to start up the device but there was only a flickering screen. I tried to switch off completely the device while the charger was connected but even that was not possible the device remained in the flickering screen state. So I decided to open the device and to detach the battery. Having in mind to reset the battery and to have the chance to at least boot into bootloader mode. Unfortunately reconnecting the battery didn't change anything. Neither booting into recovery nor booting into bootloader mode. :-( It looked like there is not enough power provided by the charger to switch on the device and the battery didn't provide any additional power. But how to charge the battery if the device goes into this flickering mode consuming all the energy provided by the charger? Even holding the power button for a longer time period (>=20sec) didn't work. The device switched off, but immediately re-entered the flickering mode ... Then I found solutions here and here which helped to gat out of this state. I tried 2 or three times to get into bootloader mode which didn't work, but then suddenly I saw at least the charging battery symbol on the screen. I disconnected / connected the charger again and the charging animation started again showing fully empty battery. I let it charge for some hours and then I was able to restart the device normally.
Good news: Even if you think the device is bricked, do not give up too early!