Add adb_keys to mainline_system if they exist

Similar to I1720644d89ec5289fbe99f95ebcdfbb3f3b20e67,
but for mainline_system. This reduces the diff between
full_base.mk and mainline_system, a step on the way to
get aosp_blueline inheriting mainline_system.

Bug: 118826166
Test: make
Change-Id: I52bfe5dfb3e1a22db2d46fe9d1f372ee25533fdd