update moisture sensor permissions

Previously thought in the discussion under ag/33001325 that it was
possible to add a normal install time permission that is rejected by
play store for reading the moisture sensor. This sensor is only used by
system components so we can update it to signature | privileged and
prevent 3p apps from accessing.

Bug: 371291781
Flag: com.android.tradeinmode.flags.trade_in_mode_2025q4
Test: th
Change-Id: I2632616f4c943bebcc979e563d01792d77e2aba1