adb_gdb gets the device fingerprint with adb shell getprop
adb_gdb gets the device fingerprint with adb shell getprop

Previously adb_gdb pulled /system/build.prop from the device to store
the build fingerprint for the pulled libraries. build.prop no longer
always contains the ro.build.fingerprint property, but it can still
be queried with adb shell getprop, so update adb_gdb to use that
instead.

Review URL: https://codereview.chromium.org/1394093002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#353550}

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