Add a warning asking user to backup device data

The "Experimental: Enable ANGLE" switch is aimed for dogfood users
and developers to test ANGLE as the system driver. On a released
commercial android device, user should not play around with the switch,
as this can brick the device into unusable state and need a factory
reset to fix. Add a warning to ask user to backup their device data
so that they are aware of the potential risk, and if they do decide
to toggle the switch, they can mitigate the risk by backing up their
device data.

This change should be backported to earlier Android release.

Flag: EXEMPT BUGFIX
Bug: b/270994705
Bug: b/459639258
Test: make, flash to Pixel 9
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:5d63abc7cb36b98b1e5e45472fdc83ccf26221fd
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:125623f0ebc3e80a8f2b4e33a7cb2bd25633971d
Merged-In: I10501b6fdee67621151aa2b206586f2024a7f95e
Change-Id: I10501b6fdee67621151aa2b206586f2024a7f95e