[Bug Fix] Add `exact string` to property qemu.hw.mainkeys

qemu.hw.mainkeys exists both in plat_property_contexts and
vendor_property_contexts. This would cause breakage in GSI build
for certain vendors. To fix, add `exact {type}` to make the property
defined in system takes precedence.

Bug: 180412668

Signed-off-by: Weilun Du <wdu@google.com>
Change-Id: I1268e6a202d561a1e43f3d71fb38c6000042306b
diff --git a/private/property_contexts b/private/property_contexts
index 7f3cb2f..7fb26fb 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -187,7 +187,7 @@
 ro.dalvik.              u:object_r:dalvik_prop:s0
 
 # qemu_hw_prop is read/written by both system and vendor.
-qemu.hw.mainkeys        u:object_r:qemu_hw_prop:s0
+qemu.hw.mainkeys        u:object_r:qemu_hw_prop:s0 exact string
 
 # qemu_sf_lcd_density_prop is read/written by both system and vendor.
 qemu.sf.lcd_density     u:object_r:qemu_sf_lcd_density_prop:s0 exact int